We told you so
When the antivirus programs (free, paid for etc) do more damage than the viruses, something is very wrong indeed.
20% performance hit on a quad core with 8GB RAM is annoying but bearable, on the other hand 40% on a netbook is intolerable.
If the machine noticeably slows down with AV running in the background then there is something amiss with the setup. Installing an SSD might help here but a polished turd is still a turd.
If nothing else this might be the nudge some people need to recycle their junk PC and get something which wasn't manufactured in the last decade.
Also scan-on-boot using 1/4 or 2/6 cores just to do that function is quite feasible, storing known good CRC32 of every file and checking it against a protected archive isn't routinely done yet.